The present invention relates to a super microfibrillated cellulose obtained by microfibrillating cellulose fibers and further microfibrillating the obtained microfibrillated cellulose to a predetermined fineness, and a process for producing the super microfibrillated cellulose.
The present invention relates also to a process for producing a coated paper and a process for producing a tinted paper, taking advantage of properties peculiar to the super microfibrillated cellulose.
When cellulose fibers such as wood pulp are microfibrillated, the fibers are divided to form fibrils which are the constituting units of the cell membranes and, therefore, the microfibrillation proceeds by branching while the fiber shape is kept to form the microfibrillated cellulose. It is known that when such a microfibrillated cellulose is added to a papermaking pulp, a paper having various interesting properties is obtained. For example, when the microfibrillated cellulose is added to a paper stock, an effect of improving the strength including tensile strength and bursting strength and also an effect of increasing the air permeability are obtained. In addition, the capacity of retaining the filler and the adsorption of a dye are also improved by the microfibrillated structure of the cellulose
It has hitherto been known that the microfibrillated cellulose can be obtained by applying a strong mechanical shearing force to cellulose fibers such as a papermaking pulp, and various processes for producing such a microfibrillated cellulose have been proposed. For example, Japanese Patent Publication No. 60-19921/1985 (corresponding to U.S. Pat. No. 4,374,702 issued Feb. 22, 1983) proposes a process for producing microfibrillated celluloses, which comprises a step of passing a suspension of a fibrous cellulose through a small-diameter orifice in which the suspension is subjected to a pressure drop of at least 3,000 psi and a high velocity shearing action followed by a high velocity decelerating impact, and a step of repeating this step until the cellulose suspension becomes a substantially stable suspension.
Japanese Patent Laid-Open No. 4-82907/1992 proposes a process for producing a fibrillated natural cellulose by breaking short fibers of natural cellulose in a dry state.
Further, Japanese Patent Laid-Open No. 6-10286/1994 discloses a process for producing microfibrillated cellulose by wet pulverization treatment of a fibrous cellulose suspension with a vibration mill containing glass, alumina, zirconia, zircon, steel or titania beads or balls as a pulverizing medium.
The above-described process proposed in Japanese Patent Publication No. 6-19921/1985 wherein the suspension of a fibrous material such as a pulp must be passed through a small-diameter orifice under a high pressure has a problem of the treatment efficiency that the solid concentration of the suspension to be processed must be kept as low as 1% by weight or below, since when a suspension having a solid concentration of above 1% by weight is passed through the small-diameter orifice, the orifice tends to be clogged. When the microfibrillated cellulose of a high concentration is to be obtained by concentrating the treated suspension having a low solid concentration, the concentration operation becomes laborious. Both the low treatment efficiency and operation efficiency cause an increase in the production cost of the microfibrillated cellulose to pose a problem that the microfibrillated cellulose produced by such a process at a high cost cannot be used for the production of products to be produced at a low cost on a large scale like a paper.
The microfibrillation in a dry state as proposed in the above-described Japanese Patent Laid-Open No. 4-82907/1992 has a problem that the obtained microfibrillated cellulose is in the form of flakes and has a low water retention, since the cellulose fibers are only slightly fibrillated, unlike those microfibrillated by the wet process.
In the wet grinding process proposed in Japanese Patent Laid-Open No. 6-10286/1994 wherein the vibration mill is used, a very long time is necessitated for the microfibrillation treatment of long fibers such as conifer fibers or non-wood fibers and, even in the treatment of short fibers such as broadleaf tree fibers, the separation of the obtained microfibrillated cellulose from beads or balls used as the pulverizing medium is difficult, since the microfibrillated cellulose thus obtained is sticky and, therefore, this process has problems in the treatment efficiency.
A process for producing a microfibrillated cellulose by solving the above-described problems has been proposed by the assignee of the present invention in Japanese Patent Laid-Open No. 7-310296/1995. This process is characterized by passing a slurry of a previously beaten pulp through a rubbing part of a rubbing apparatus comprising two or more grinders each comprising abrasive grains having a grain size of No. 16 to 120 to microfibrillate the pulp and thereby to obtain microfibrillated cellulose having an arithmetic average fiber length of 0.05 to 0.3 mm, a water retention value of at least 250%, and a rate of the number of fibers not longer than 0.5 mm of at least 95% based on the total number of the fibers as calculated by adding up. This process has an advantage that even when the solid concentration is as relatively high as about 5 to 6% by weight, the microfibrillation treatment can be efficiently conducted, since the pulp in the slurry to be fed into the rubbing apparatus has been previously beaten.
Not only the various production processes described above but also the uses of the microfibrillated cellulose have been already developed. Japanese Patent Laid-Open No. 4-194097/1992 proposes a coated paper produced by adding the microfibrillated cellulose to a coating material for size press or the like and then coating at least one surface of a paper with the coating material. However, according to our tests wherein the microfibrillated cellulose was added to a coating material comprising starch and other ingredients and the obtained coating material was applied to a paper to form a coated paper, it was found that this process has problems that the coating material was thickened, that the microfibrillated cellulose aggregated to some extent to make the uniform coating impossible and to realize a foreign matter feeling or to form a streak trouble and to cause faults in the coating, and that the printability of the coated paper is impaired. After intensive investigations made for the purpose of finding the causes of the problems, we have found that the fiber length distribution of the microfibrillated cellulose is improper and that the water retention value is excessively low.
In Japanese Patent Laid-Open No. 7-324300/1995 the assignee of the present invention previously proposed a process for producing a tinted paper by adding a carrier carrying a dye or pigment, prepared by supporting the dye or pigment on a microfibrillated cellulose, to a paper stock prepared mainly from a papermaking pulp and manufacturing paper from the resultant mixture. It was found that even by this process, the level tinting is impossible when microfibrillated cellulose having a size larger than a predetermined size is contained in the mixture, and the tinted paper product having a very fine, unevenly dyed portions is obtained.